The technical field of this invention is an occupant weight detection system for a vehicle seat with a foam seat cushion.
Vehicle occupant restraint device controls are now being designed to distinguish seat occupants by weight and use this information in determining, in the case of a crash, whether or not to deploy a restraint and, if so, with what force. One type of vehicle seat occupant weight detecting system uses a force or pressure sensor under a foam seat cushion on which an occupant sits; and a particular type of force or pressure sensing apparatus is a non-compressible fluid filled bladder comprising an array of cells in fluid communication with each other and a pressure sensor connected to the interior of the bladder to measure the fluid pressure therein.
Systems of this type have had to be refined to accurately distinguish between two or more classes of vehicle occupants defined in certain governmental regulations; and this refinement involves, among other things, compensation for certain mechanical and/or environmental effects inherent in the apparatus that tend to distort the output pressure signal from that indicating the true weight of the occupant. It has been discovered that one of these effects is relative humidity within the foam material of the seat cushion, which affects the force/pressure transmitting properties of the foam and thus the relationship between the weight of a seat occupant and the pressure exerted on pressure sensing apparatus under the seat cushion. Although the prior art includes references to correcting the output of occupant weight sensors for humidity of the air, such references deal with systems having sensors, such as open capacitive sensors or ultrasound reflective sensors, in which the sensor itself is sensitive to humidity. It has not been known in the prior art that it would be necessary or advantageous to correct for humidity in the seat foam when the sensor was itself not significantly affected by humidity.
Testing has shown that there can be a significant time delay between a change in relative humidity of the air adjacent the foam and a change in the humidity level within the foam. The foam material includes a large number of very small air pockets, only a small proportion of which are close to the outer surface of the foam cushion. In addition, the seat foam is usually covered with a material that further impedes air flow in and out of the foam. Exchanges in air flow between the seat foam and the atmosphere are propelled by xe2x80x9cfoam activityxe2x80x9d: that is, compression of the foam to expel a portion of the air within, followed by release of the compression to allow the foam to expand and pull in external air. In the absence of xe2x80x9cfoam activityxe2x80x9d it can take a very tong, time for the average humidity within the foam to adjust to that outside the foam; but in the presence of such activity, the response can be significantly faster.
Furthermore, some foam seat cushions tend to exhibit a cross-correlation effect between relative humidity and temperature, most probably due to the facts that (1) the humidity effect on a foam seat cushion appears to vary with the absolute amount of water in the foam and (2) the relationship between the measured relative humidity and the absolute humidity in air can vary in a strongly non-linear manner with changes in temperature. In some cases, the cross-correlation effect can be as great as the effect of humidity alone.
The invention described and claimed herein provides a humidity compensated vehicle seat occupant classification system with pressure responsive apparatus adapted for engagement with an underside of a foam seat cushion so as to respond to a weight of an occupant on an upper side of the foam seat cushion and generate a pressure signal therefrom. In the system, a humidity sensor is adapted to respond to relative humidity of air adjacent the pressure responsive apparatus to generate a humidity signal. The humidity signal is used to compensate at least one of the following: the pressure signal, a stored reference pressure value and/or a stored threshold value. The occupant classification is determined at least partly in response to the compensated one of the pressure signal, the stored reference pressure value and the stored threshold value. In a preferred embodiment, a stored reference pressure value representing an empty seat pressure is compensated; and in another preferred embodiment, the stored threshold value is also compensated.
Preferably, the system includes a time delay between the reading of a humidity value and the full use of that humidity value in compensation, and the time delay is preferably variable according to an activity factor of the foam seat cushion derived from dynamic variations in the pressure signal associated with a pumping action on the foam tending to increase the rate of exchange of air between the foam and the atmosphere outside the foam. In a preferred embodiment, the activity factor is derived by counting consecutive, alternating excursions of signal magnitude above a first predetermined level and below a second predetermined level lower than the first predetermined level.
Preferably, the temperature adjacent the foam seat cushion or the pressure sensor is further used with the humidity signal to generate a cross-correlation value for further compensating whichever of the pressure signal, the stored reference pressure value and the stored threshold value is humidity compensated. This cross-correlation value may be derived from a product of tile humidity and temperature signals and may further be derived from a product of the humidity signal and the square of the temperature signal.